Control of blowdown in steam boilers

ABSTRACT

Blowdown of a pressurised steam boiler heated by a burner  1  and including a blowdown flow control valve  17  is controlled by opening the control valve in response to receiving a control signal from a control unit  20  to allow water to leave the boiler. The control unit receives one or more signals indicative of the steam production rate of the boiler and adjusts the amount of blowdown in dependence upon the aggregate steam production.

TECHNICAL FIELD

This invention relates generally to blowdown of steam boilers heated byburners. More particularly, the invention relates to control systems foruse in such installations and to methods of commissioning and operatingsuch installations. The invention relates in particular, but notexclusively, to a steam boiler such as might be used in hospitals,hotels, offices or other industrial, commercial or domestic premises.

BACKGROUND OF THE INVENTION

Steam boilers are well known. Examples of US patents referring toaspects of the design of pressurised steam boilers are U.S. Pat. No.6,520,122 and U.S. Pat. No. 7,249,573, the contents of which areincorporated herein by reference.

Standard steam boilers are fitted with a bottom blowdown valve which,when opened for a short period of time allows an amount of hot water toleave the boiler, carrying with it solid deposits that have accumulatedat the bottom of the boiler as it has been operating. The removal ofsuch solids is important to ensure that the heat transfer efficiency ofthe boiler is maintained. It is therefore conventional practice toeffect a bottom blowdown operation at regular intervals, for exampleonce every 8 hours. For a boiler producing about 20,000 lbs (about 10tonnes) of steam an hour, a blow down of the order of 10 to 15 secondswould be typical. During the blow down a substantial amount of wasteheat passes out from the boiler in the water that is blown down. It isgenerally accepted that 2 to 3 percent of the total heat input to aboiler is lost through such blow down operations. Given that a boiler ofthe size indicated might use of the order of £500,000 or $800,000 offuel in a year, that 2 to 3 per cent of extra fuel to support the blowdown operations is very significant.

It is an object of the invention to provide an improved method ofcontrolling blowdown of a boiler and an improved steam boiler.

SUMMARY OF THE INVENTION

According to the invention there is provided a method of controllingblowdown of a steam boiler heated by a burner, the boiler comprising ablowdown flow control valve that can be opened in response to receivinga control signal from a control unit to allow water to leave the boiler,wherein the control unit receives one or more signals indicative of thesteam production rate of the boiler and adjusts the amount of blowdownin dependence upon the aggregate steam production of the boiler.

By adjusting the amount of blowdown in dependence upon the aggregatesteam production of the boiler, it becomes possible to tailor the amountof blowdown to that which is needed. That can avoid in particularoperating with excessive blowdown when the boiler is operating wellbelow its maximum continuous rating and can enable significant fuelsavings to be made, and also reduce wastage of water.

Typically, the control unit opens and closes the control valve amultiplicity of times over a time period. For example it may open andclose the control valve three times each day. Each opening and closingof the control valve results in a blowdown operation. Accordingly, thecontrol unit may open and close the control valve a multiplicity oftimes over a time period, causing a multiplicity of blowdown operations.It is possible to adjust the amount of blowdown in a variety of ways,for example, by adjusting the frequency of the blowdown operations, butit is preferred that, in order to adjust the amount of blowdown, thecontrol unit adjusts the amount of water blown down from the boiler ineach blowdown operation.

There are also various ways in which the amount of water blown down fromthe boiler in each blowdown operation can be controlled; for example,the degree of opening of the blowdown control valve may be controlled;it is preferred, however, that the control unit adjusts the amount ofwater blown down from the boiler in each blowdown operation by adjustingthe time for which the control valve is open during each blowdownoperation.

Where reference is made to “signals indicative of the steam productionrate of the boiler”, it should be understood that those signals may takevarious forms which may be directly or indirectly related to the rate ofsteam production. For example, the signals may be derived frommeasurement of many different variables, including but not limited tothe following: steam flow rate from the boiler; amount of fuel fed tothe burner; amount of air fed to the burner; amount of exhaust gasesgenerated. Generally, since the amount of evaporation, which correspondsto the amount of steam produced, is that most closely related to theoptimum amount of blowdown, it will be preferred, for greatest accuracy,that it is the aggregate steam production since the previous blowdownthat is assessed.

The method preferably further includes an initial step of setting ablowdown amount for a given steam production rate of the boiler, thecontrol unit thereafter adjusting the amount of blowdown according to acomparison of an actual steam production amount of the boiler with thatwhich would apply for the given steam production rate. The given steamproduction rate of the boiler is preferably the maximum continuous rateof steam production of the boiler. The initial setting step ispreferably carried out during commissioning of the boiler. By settingthe amount of blowdown at the time of commissioning, the opportunity isprovided for a commissioning engineer to consider the appropriatesetting at the maximum continuous rate of steam production of theboiler, taking all known factors into account.

The steam production rate may be measured directly by a steam flow meterwhich may provide a signal representing that rate to the control unit,which can then integrate that rate to arrive at the aggregate steamproduction. Another option is to measure the rate of steam productionindirectly, for example as described in U.S. Pat. No. 6,520,122.

The time for which the control valve is open during each blowdownoperation may be adjusted downwardly in dependence upon the reduction ofthe aggregate steam production from that which would apply for themaximum continuous rate of the boiler. Even a crude adjustment can beeffective in reducing heat loss. Preferably, the time is adjusteddownwardly approximately in proportion to the reduction in the aggregatesteam production.

According to another aspect of the invention, there is provided apressurised steam boiler including:

a boiler housing for containing water in the boiler,

a burner for heating water in the boiler and converting the water intosteam,

a blowdown control valve that is openable to allow water to leave theboiler in a blowdown operation, and

a control unit for controlling the opening of the blowdown controlvalve, the control unit being connected to receive one or more signalsindicative of the steam production rate of the boiler and to adjust theamount of blowdown in dependence upon the aggregate steam production ofthe boiler.

The control unit may be arranged to open and close the control valve amultiplicity of times over a time period, causing a multiplicity ofblowdown operations, and, in order to adjust the amount of blowdown, thecontrol unit may be arranged to adjust the amount of water blown downfrom the boiler in each blowdown operation.

The control unit may be arranged to adjust the amount of water blowndown from the boiler in each blowdown operation by adjusting the timefor which the control valve is open during each blowdown operation.

The control unit may further include a store arranged to store pairs ofvalues of air and fuel valve settings at various firing rates of theburner and the control device may be arranged, when the firing rate ofthe burner is to be changed, to control the air and fuel valve settingsin dependence upon the stored air and fuel valve settings. A burnercontrol unit of this kind is described in GB2138610A, the contents ofwhich is incorporated herein by reference.

Where reference is made to a “control unit”, it should be understoodthat such a unit may comprise one physical entity or two or moreentities. In an illustrated embodiment of the invention described below,the control unit is a single physical entity, but as explained in thedescription below, it is possible for there to be two or more separatecontrol modules.

It will be appreciated that the method of controlling blowdown of theboiler and the steam boiler are closely related to each other and thatfeatures described in respect of the method may be adopted in the steamboiler and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example an embodiment of the invention will now be describedwith reference to the accompanying schematic drawing, of which:

FIG. 1 is a block diagram of a pressurised steam boiler installation.

DETAILED DESCRIPTION OF EMBODIMENTS

Referring to FIG. 1, a pressurised steam boiler installation generallycomprises a fuel burner 1, which in this case is a gas and oil burner,and a boiler housing 3. Gas is fed along a pipe 2 via a butterfly valve4, and/or oil is fed along a pipe 5 via a butterfly valve 6. Air isdriven along a duct 7 by a fan 8 via a damper valve 9. The burner alsohas a pilot fuel feed 10.

In the burner 1, the fuel and air are mixed and combustion takes place.The products of combustion pass from the burner 1 through a heatexchanger 11 in the boiler housing 3 containing water 14 into the bottomof a stack 15. The combustion products pass up the stack 15 and into theatmosphere.

An outlet pipe 16 extends from the bottom of the boiler housing 3 via ablowdown flow control valve 17 to a blowdown receiver (not shown),providing a bottom blowdown facility that is in most respectsconventional, but employs an electrically controlled blowdown valve andservomotor assembly. The use of an electrically controlled blowdownvalve is unconventional. In order to ensure operation, even in the eventof a power failure, the assembly is also connected to a battery supplyand, in the event of a power failure during blowdown, the blowdown valveis closed under battery power.

A control device 20 is provided to control the operation of theinstallation. The control device receives many inputs and controls theoperation of many parts of the installation in a conventional manner; inthe interests of clarity, it is only the control aspects of moresignificance to the present invention that will now be described andthat are shown in FIG. 1. The butterfly valve 4 controlling the flow ofgas to the burner 1 is set by a servo motor 21 connected to the controldevice 20 and able to receive control signals determining the positionadopted by the servo motor 21. The butterfly valve 6 controlling theflow of oil to the burner 1 is set by a servo motor 22 connected to thecontrol device 20 and able to receive control signals determining theposition adopted by the servo motor 22. The damper valve 9 controllingthe flow of air to the burner is set by a servo motor 23 connected tothe control device 20 and able to receive control signals determiningthe position adopted by the servo motor 23. The bottom blowdown valve 17is controlled by a servo motor 24 connected to the control device 20 andable to receive control signals for opening and closing the valve 17.

The control unit 20 includes a store 30 which is also shown in FIG. 1 inan expanded schematic form. In the expanded part of FIG. 1, the store 30is shown with the left hand column, A, showing the numbered rows fordifferent sets of values. There are then two further columns, B and C,which store the settings of the fuel and air valves, as described inmore detail in GB 2138610A. Pairs of air and fuel valve settings arestored for different firing rates of the burner. Those settings aregenerated by a commissioning engineer when the control system for theburner is first set up.

In operation, when the control system of FIG. 1 is commissioned, thecommissioning engineer sets the air and fuel valves to settings thatprovide optimum combustion conditions at a given firing rate of theburner. Once the engineer is satisfied that the best settings have beenachieved for a given firing rate, they are stored in the store 30. Thecommissioning engineer can then adjust the firing rate of the burnerupwards or downwards and store a set of optimum values for that firingrate. By repeating that process, values can be entered across the fullfiring range of the burner. If the burner is to operate on only one fuelthen the commissioning can be carried out just with that fuel, but ifthe burner is also to operate with a second fuel, the commissioningprocedure described above can be repeated for the second fuel.

The commissioning engineer also sets the time period for which theblowdown valve 17 is opened when the boiler is operating at its maximumcontinuous rating; that setting, referred to herein as the “MCR blowdowntime” can take account of all relevant factors known to thecommissioning engineer, including for example measurements of waterquality that may have been provided by a water analyst. Usually theboiler will be set to perform a blowdown at set times which thecommissioning engineer does not vary; for example, in this particularexample a boiler operates for three 8 hour shifts each day and oneblowdown is performed at the end of each shift. A typical time periodfor which the blowdown valve 17 is opened when the boiler is operatingat its maximum continuous rating is 12 seconds, but as just explained,this time period is chosen by the commissioning engineer.

Once the installation has been fully commissioned, it is ready foroperation. When the burner is set to a given firing rate, the controldevice looks up in the store 30 the settings of the appropriate servomotors 21, 22 and 23 for that firing rate and adjusts them accordingly.When the time comes for the control device to open the blowdown valve 17at the end of a shift, it calculates the aggregate steam production ofthe boiler during the shift; in a case where the boiler has beenoperating throughout the shift at its maximum continuous rating, thenthe blowdown valve 17 is opened by the control device 20 for the “MCRblowdown time”. In the common case, however, when the burner has beenoperating at a lower average firing rate and the aggregate steamproduction has therefore been lower, the blowdown valve 17 is opened bythe control device 20 for only a part of the “MCR blowdown time”, inproportion to the reduction in the rate of steam production. For examplewhen the boiler has been operating at an average of 25% of maximumcontinuous rating, the blowdown valve 17 is opened by the control device20 for only 25% of the “MCR blowdown time”; thus, if the “MCR blowdowntime” is 12 seconds, the blowdown valve would in this case be opened for3 seconds only. As will be understood the aggregate steam production canbe ascertained by the control device 20 in various ways. For example theamount of steam generated by the boiler can be measured by a steam flowmeter or calculated, for example as described in particular in columns11 and 12 of U.S. Pat. No. 6,520,122, the description of which isincorporated herein by reference. The control device can then apply aproportionate adjustment to the time for which the valve 17 is openedusing conventional proportionate control techniques.

Whilst one particular example of the invention has been described, manymodifications may be made to it without departing from the invention.Some such modifications will now be explicitly mentioned, but it shouldbe understood that many others may be made.

In the particular example described above, the installation runs inshifts of 8 hours each with one blowdown at the end of each shift. Itwill be understood that the invention may be employed with differentlengths of shift and/or with each blowdown in the middle of a shiftand/or with more than one blowdown in a shift. It is also possible thatwhen the boiler is operating at its maximum continuous rating, it isarranged to have two or more blowdowns which differ in length; if thatwere the case, then at a lower steam production rate the duration of oneor more of those blowdowns would be reduced in order to reduce the totalamount of blowdown at a lower steam production rate.

In the embodiment described the blowdown valve is opened fully and thetotal amount of blowdown controlled by controlling the time for whichthe valve is fully opened; an alternative possibility would be to alterthe degree of opening of the valve instead of or in addition to alteringthe time for which it is open.

In the embodiment of the invention described above, it is the steamproduction rate that is directly or indirectly measured. That isadvantageous in that it is the steam production rate that is mostclosely related to the optimum amount of blowdown, but it should beunderstood that another possibility is to measure the firing rate of theburner and use the average firing rate to determine the amount ofblowdown. The firing rate may be measured by monitoring the fuel flowrate to the burner.

In the illustrated embodiment, a single control unit is shown in whichall the control functions are carried out. It should be understood,however, that it is within the scope of the invention for the “controlunit” to be provided by two or more control modules which may bephysically separate from one another. For example, there may be aphysically separate control module for the blowdown valve 17, and thatmodule may adjust the time for which the valve 17 is open according to asignal it receives from another control module and/or some other devicesuch as a steam flow meter. In that way the control of the amount ofblowdown can be carried out independently of an overall control systemfor the boiler installation.

It is possible for other values to be stored in the store 30 alongsidethe settings of the air and fuel valves. For example in WO2012/056228A2,the contents of which are incorporated herein by reference, a modifiedversion of the control arrangement of GB2138610A is described in whichrespective values of fuel pressure and air pressure upstream of theburner are stored in the store 30 for each pair of fuel and air valvesettings.

In GB 2169726A, the contents of which are incorporated herein byreference, the control device 20 is connected to receive a feedbacksignal from an exhaust gas analysis system and that signal is used totrim the air valve setting from the stored value to a slightly differentvalue. That arrangement, with or without the modifications anddevelopments described in WO2012/056228A2, may be employed inembodiments of the present invention.

Where in the foregoing description, integers or elements are mentionedwhich have known, obvious or foreseeable equivalents, then suchequivalents are herein incorporated as if individually set forth.Reference should be made to the claims for determining the true scope ofthe present invention, which should be construed so as to encompass anysuch equivalents. It will also be appreciated by the reader thatintegers or features of the invention that are described as preferable,advantageous, convenient or the like are optional and do not limit thescope of the independent claims.

1. A method of controlling blowdown of a steam boiler heated by aburner, the boiler comprising a blowdown flow control valve configuredto open in response to receiving a control signal from a control unit toallow water to leave the boiler; wherein the control unit receives oneor more signals indicative of the steam production rate of the boilerand adjusts the amount of blowdown in dependence upon aggregate steamproduction of the boiler.
 2. A method according to claim 1, wherein thecontrol unit opens and closes the control valve a multiplicity of timesover a time period, causing a multiplicity of blowdown operations, and,in order to adjust the amount of blowdown, the control unit adjusts theamount of water blown down from the boiler in each blowdown operation.3. A method according to claim 2, wherein the control unit adjusts theamount of water blown down from the boiler in each blowdown operation byadjusting the time for which the control valve is open during eachblowdown operation.
 4. A method according to claim 3, wherein time forwhich the control valve is open during each blowdown operation isadjusted downwardly approximately in proportion to a reduction in theaggregate steam production.
 5. A method according to claim 1, furtherincluding an initial step of setting a blowdown amount for a given steamproduction rate of the boiler, the control unit thereafter adjusting theamount of blowdown according to a comparison of an actual steamproduction amount of the boiler with a steam production amount thatwould apply for a given steam production rate.
 6. A method according toclaim 5, wherein the given steam production rate of the boiler is amaximum continuous rate steam production of the boiler.
 7. A methodaccording to claim 5, wherein the initial setting step is carried outduring commissioning of the boiler.
 8. A steam boiler comprising: aboiler housing for containing water in the boiler; a burner for heatingwater in the boiler and converting the water into steam, a blowdowncontrol valve that is openable to allow water to leave the boiler in ablowdown operation, and a control unit for controlling opening of theblowdown control valve, the control unit being connected to receive oneor more signals indicative of a steam production rate of the boiler andto adjust the amount of blowdown in dependence upon aggregate steamproduction of the boiler.
 9. A boiler according to claim 8, wherein thecontrol unit is arranged to open and close the control valve amultiplicity of times over a time period, causing a multiplicity ofblowdown operations, and, in order to adjust the amount of blowdown, thecontrol unit is arranged to adjust the amount of water blown down fromthe boiler in each blowdown operation.
 10. A boiler according to claim9, wherein the control unit is arranged to adjust the amount of waterblown down from the boiler in each blowdown operation by adjusting timefor which the control valve is open during each blowdown operation. 11.A boiler according to claim 8, wherein the control unit further includesa store arranged to store pairs of values of air and fuel valve settingsat various firing rates of the burner and the control device isarranged, when a firing rate of the burner is to be changed, to controlthe air and fuel valve settings in dependence upon stored air and fuelvalve settings.